Colour projector

ABSTRACT

In a projector using a digital image generator of rectangular form, such as a digital micro-mirror device (DMD), the input light beam striking the image generator defines a beam section of greater sectional area than an intended circular beam, so as to increase the number of digital elements of the image generator contributing to the light beam output. An optical arrangement, such as a mask, defines the beam section, which may be elliptical, or pincushion like, rather than circular. A lens, or other optical component, corrects the beam shape produced by the mask to the circular beam shape. Alternatively, a fiber-optic bundle replaces the mask and lens arrangement.

This invention is directed to the creation of lighting effects, and inparticular aspects to the use in creating lighting effects of digitalimage projectors such as those used widely for graphics or videoprojection.

The image generator or modulator used in known digital image projectorstakes a wide variety of forms employing known digital imagingtechniques. A particularly convenient technology used in graphics orvideo projection is the digital micro-mirror technology of TexasInstruments (DMD).

A characteristic of the DMD and most other digital image generationtechnology is an inherently rectangular geometry often linked to a 4:3or 16:9 aspect ratio common to video display. It is of course common instage lighting and similar applications to require a beam of circularsection. Whilst a circular image can of course be digitally created,this brings unexpected problems.

It is hence an object of the invention to provide a improved projectionsystem for such stage lighting applications.

Accordingly, in one aspect the invention therefore consists in aprojector for producing a light beam of substantially circular section,comprising: a digital image generator of substantially rectangular form;and an optical arrangement associated with the image generator, theoptical arrangement defining a beam section of greater sectional areathan the intended circular beam, such as to increase the number ofdigital elements of the image generator contributing to the light beamoutput, and correcting the beam shape to a circular beam shape.

In one form of the invention, the optical arrangement comprises a maskfor defining the beam section, and a lens for correcting the beam shapeto the circular beam shape. In another form, the optical arrangementcomprises a fibre-optic bundle.

The use of a beam section larger than the circular beam allows more ofthe pixels of the DMD to be used to produce the output, creating ahigher intensity image, or increasing efficiency.

Another aspect of the invention consists in a projector for producing alight beam of defined section, comprising a digital image generator ofrectangular form and an optical mask arrangement associated with theimage generator, the mask arrangement having an inactive configurationin which the beam section is governed by the image generator and anactive configuration in which the beam section is governed by the mask.

Preferably, the defined section is circular.

The significant advantage of using an optical mask to define the(usually) circular section of the beam, is that full black can beachieved at the beam perimeter, rather than the “video black” which isachievable from the image generator itself.

Suitably, the mask has a window, for defining the beam section, ofeither elliptical or pincushion shape. The principal axes of the ellipseshould typically be aligned with the axes of the rectangular imagegenerator.

Advantageously, the optical means comprises a cylindrical lens.

Suitably, image data received by the image generator is distorted topre-compensate for the effect on the image of the optical means.

Numerous methods for image projection are of course known in the art. Ingeneral, two distinct methods for the introduction of colour intoprojectors are known. First, a combination of projectors or projectionelements, each producing a different colour, is employed, in order tocreate the desired effect. For example, the output from separate red,green and blue projectors may be combined to form a colour image, theRGB components of the image being separated and sent to the respectiveprojectors.

Second, a single projector is used. Either an image generator is usedwhich is inherently full colour or, more usually, a single, monochromeimage generator is multiplexed between red, green and blue illumination.For example, a colour wheel may be employed having segments for thedifferent colours, with the image generator being supplied with therespective colour separations, synchronously with red, green and blueillumination.

It has been recognized by the inventors that such projectors can have anentirely different use in theatrical lighting. Here, the projected imagemay include graphics or video, with the versatility of the digital imagegenerator enabling a wide range of stationary or moving patterns to begenerated, in ways which would be very difficult or impossible withconventional stage lighting. At other times, such digital projectors maybe needed to create more traditional effects, such as the monochromeillumination of actors and stage sets with a plain beam or with a maskor gobo effect created using the power of the digital image generator.

It is therefore an object of a further aspect of the invention toprovide a projection system which is capable of providing a broaderrange of effects.

Accordingly, the present invention consists in one aspect in a colourprojection device comprising a light source, a plurality of lightmodulating means and colour filtering means associated with eachrespective light modulating means and operating on respective portionsof the colour spectrum, whereby the respective light modulating meansand colour filtering means cooperate to form a combined output, whereinthe colour filtering means is actuatable between active and inactivepositions, whereby light from the light source encountering a respectivelight modulating means is modulated when the respective colour filteringmeans is in the active position.

This system therefore has the significant advantage that, in oneexample, red, green and blue colour filters may be removed from therespective light modulators, allowing unfiltered light from all threelight modulators to form an output picture having much greaterintensity.

Suitably, a colour filtering means associated with a given lightmodulating means provides a plurality of different colours for outputlight. Preferably, the plurality of colours comprises a primary colourspecific to that light modulator, and a range of custom colours commonto all light modulators. In this way, there is added to the options of acolour image and a high intensity white beam, the additional option of ahigh intensity monochrome beam according to any one of the customcolours. Conveniently, the colour filtering means comprises a colourwheel.

A further aspect of the invention consists in a colour projection devicecomprising three image generators each adapted to receive a respectivecolour separated digital image signal; and an optical arrangement suchthat the separate image generators operate on different regions of thecolour spectrum to deliver in combination a projected colour image;characterized in that the optical arrangement is further provided with alighting mode in which the three image generators operate on the sameregion of the colour spectrum to deliver a high intensity monochromeimage beam.

The invention will now be described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 is a diagram illustrating a light modulator and filter mask of aprojector according to an embodiment of the invention;

FIG. 2 is a diagram of a filter mask according to an embodiment of theinvention;

FIG. 3 is a diagram illustrating a light modulator, filter mask and lensarrangement of a projector according to an embodiment of the invention;

FIG. 4 is a diagram of a filter mask according to another embodiment ofthe invention.

FIG. 5 is a diagram of a single light modulator of a three-chip typeprojector according to an embodiment of the invention.

FIG. 6 is a diagram illustrating a colour filter according to anembodiment of the invention; and

FIG. 7 is a diagram of a colour wheel according to an embodiment of theinvention.

FIG. 1 shows a DMD projector having a filter mask system according to anembodiment. Light (110) from a light source (not shown) is received andreflected by the DMD (120). The image generated on the DMD is thusincorporated into the light beam to produce an image bearing beam (125).This image bearing beam then passes through a filter mask (130) toproduce the projection beam (140) which continues towards the projectionlens (not shown) to produce the final output.

An embodiment of the invention is illustrated in FIG. 2. It is a commonrequirement of stage and other lighting effects that the projected beambe circular. A typical mask (200) for a colour filter, as shown at “a”in FIG. 2 will therefore have a circular window (210). Since DMD's (andmost digital image generators) are typically rectangular, such a maskrestricts the output of the DMD and enables full black to be achieved atthe beam perimeter. This mask is conveniently located in essentially thesame plane as the image generator itself.

In one embodiment of the invention, the window is elliptical (220) inorder that a greater percentage of the discrete mirror elements of theDMD contributes to the beam. Subsequently, in order to correct theellipsoidal effect, the beam is passed through an anamorphic lens,producing a circular beam profile (230).

Such an arrangement is illustrated in FIG. 3. The light (110) isreflected from the DMD (120) and passes through the filter mask (130),which has a non-circular window. The output beam (140) is then opticallycorrected by the lens (300), to produce the output projection beam(310), having the desired circular profile (230).

The image data input to the DMD may be pre-processed in order tocompensate for the effect on the image of this change in beam shape. Forexample, the image may be distorted to fit an elliptical shape, thus theimage will itself be corrected by the lens (300) to its non-distortedform.

Pixels in the centre of the image will typically not be significantlycorrected, but pixels at the lateral edges of the image may be combinedor overlapped with several neighbouring pixels in the corrected outputimage. In one embodiment, the processing alters the gray-scale of areasof the image which will be corrected more, so that once the correctedimage is formed, these areas do not appear darker or lighter than therest of the image, due to the combination or overlapping of pixels fromthe enlarged elliptical window.

Other shapes may also be employed in the window of the mask in order toincrease the total light output. For example, the so-called “pincushion”shape of the window (400) of FIG. 4 may be used. Optical means is thenemployed to return the beam section to circular form, though this mayrequire a series of lenses, rather than a single anamorphic lens.Typically, the image data will be pre-processed to compensate for thecorrection.

It should be noted that the invention is not restricted to theproduction of a circular beam. A variety of beam shapes may be employed,with a variety of optical means employed to increase the number of DMDpixels contributing to the output.

For example, a fibre-optic bundle may be employed to replace the maskand lens arrangement. An input end of the bundle is positioned beforethe DMD and arranged in a rectangular ferrule, each fibre associatedwith a pixel of the DMD. At the output end, the fibres are arranged in acircular (or other shaped) ferrule.

In one embodiment, the fibres are arranged randomly between the inputand output ferrules, so that pixels of the image input to the DMD are nolonger in the correct position in the image beam, producing a novellighting effect.

Furthermore, the invention is not restricted to use of DMDs; any of thedigital image generation devices known to the art may be employed.

In a further embodiment of the invention, the example is used of aprojector having three DMD light modulators—a so-called three chipsystem although it should be noted that a variety of projector types maybe used in embodiments of the invention.

FIG. 5 shows for convenience a single DMD of a three-chip projectorhaving a filter system according to an embodiment. Light (110) from alight source (not shown) is passed through a filter (500), producing acoloured beam (510). This is received and reflected by the DMD (120).The image generated on the DMD is thus incorporated into the light beamto produce an image bearing beam (520), which continues towards theprojection lens (not shown) to produce the final output.

In this diagram, the filter is a red (R) filter, producing the redcomponent of the colour picture to be produced by the projector. Thustwo other DMDs, one having a green filter, the other a blue filter, areincorporated into the apparatus, functioning in the same way. The RGBoutputs are then combined to form the colour image output of theprojector.

Also in this diagram, the filter (500) is positioned before the DMD(120). In such embodiments, the DMD is prevented from further excessiveheating, as the filter removes some of the light intensity. However, inalternative embodiments, the filter is positioned in the path of lightexiting the DMD.

Referring to FIG. 6, each filter (500) may be moved from an engagedposition (601) to a disengaged position (602) in front of the DMD (120).This allows the output of the particular DMD to be either the usualcolour (R, G or B) output or a non-filtered output. In one embodiment ofthe invention, this engaged/disengaged shutter type arrangement is usedwhen a white light output is required in addition to the normal colouredimage. If all three filters are removed from the respective DMD chips,no colour will be present in the output, but a high intensity white beamwill be formed, carrying in monochrome any image (or the aggregate ofany image) defined by the DMD.

The increase in intensity from utilising all three chips may be largeenough that the desired output intensity can be achieved using lowerresources. For example, the light source could simply be dimmed formonochrome projection, lowering power consumption. The chips could beactuated for shorter periods, again saving power, and reducing heatintensity and wear on the DMDs over long periods of use.

In the embodiment shown in FIG. 6, the filter (500) is engaged anddisengaged by means of a shutter (650) moved back and forth in front ofthe DMD. This shutter could be activated by any simple mechanical orelectronic means known to those skilled in the art.

In certain embodiments, the shutters for each of the RGB filters may belinked, or at least cooperatively actuated, so that the picture iseither full colour, with all filters engaged, or full black/white, withno filters engaged.

Further effects may be achieved through use of colour wheels, such asthat shown in FIG. 7. The example shown in FIG. 7 is a wheel (795)separated into red (760) green (770) blue (780) and “white” (790)quadrants. The “white” quadrant may be a colourless filter, or merely ahole in the colour wheel. In the “normal” mode, the colour wheels forthe respective DMD chips are set to R, G and B respectively. For ablack/white beam, all three wheels are set to “white”. Additionally, fora monochrome red (or blue or green) beam, all three wheels are set tored (or blue or green). In a preferred form, each colour wheel (oralternative arrangement for bringing colour filters into the opticalpath) will comprise:

-   -   one section taking the primary colour specific to the image        generator;    -   one transparent or “white” section; and    -   a plurality of custom colours which are common to all image        generators.

The custom colours common to each wheel allow the creation of monochromebeams, with all three wheels set to the custom colour, in colours otherthan the primary colours on the wheel.

Colour wheels may, in certain applications, make inefficient use ofinput light. It has therefore been proposed, for example in U.S. Pat.No. 6,324,006, that part of the light which falls on the colour wheel,but which is not used to produce the output, may be recaptured. Suchmethods of recapture or reuse of otherwise wasted light will findapplication in aspects of the present invention, not least thosedirected to maximizing the output intensity.

It will be appreciated by those skilled in the art that the inventionhas been described by way of example only, and a wide variety ofalternative approaches may be adopted. In particular, it should be notedthat the various embodiments described may be employed in combination.For example, the filter masks described with reference to FIGS. 1 to 4may be employed with a single-chip projector, or with a three-chipprojector, either of which employing the colour filters as describedwith reference to FIGS. 5 to 7.

1. A projector for producing a light beam of substantially circularsection, comprising: a digital image generator of substantiallyrectangular form; and an optical arrangement associated with the imagegenerator, the optical arrangement defining a beam section of greatersectional area than the intended circular beam, such as to increase thenumber of digital elements of the image generator contributing to thelight beam output, and correcting the beam shape to a circular beamshape.
 2. A device according to claim 1, wherein image data received bythe image generator is are distorted to pre-compensate for the effect onthe image of the optical arrangement.
 3. A device according to claim 1,wherein the optical arrangement comprises a mask for defining the beamsection, and a lens for correcting the beam shape to the circular beamshape.
 4. A device according to claim 1, wherein the optical arrangementcomprises a fiber-optic bundle.
 5. A projector for producing a lightbeam of defined section, comprising a digital image generator ofrectangular form and an optical mask arrangement associated with theimage generator, the mask arrangement having an inactive configurationin which the beam section is governed by the image generator and anactive configuration in which the beam section is governed by the mask.6. A projector according to claim 5, wherein the section is circular. 7.A color projection device according to claim 1, wherein the mask has awindow, for defining the beam section, of substantially ellipticalshape.
 8. A color projection device according to claim 1, wherein themask has a window, for defining the beam section, of substantiallypincushion shape.
 9. A color projection device comprising a lightsource, a plurality of light modulators and a color filter associatedwith each respective light modulator and operating on respectiveportions of the color spectrum, whereby the respective light modulatorand color filters cooperate to form a combined output, wherein the colorfilters are actuatable between active and inactive positions, wherebylight from the light source encountering a respective light modulator ismodulated when the respective color filter is in the active position.10. A color projection device according to claim 9, wherein therespective color filters produce the colors red, green, and blue,respectively.
 11. A color projection device according to claim 9,wherein a color filter associated with a given light modulator providesa plurality of different colors for monochrome output light.
 12. A colorprojection device according to claim 10, wherein the color filtercomprises a color wheel.
 13. A color projection device according toclaim 12, wherein the active position is achieved when colored sectionsof the wheel produce the output light, and wherein the inactive positionis achieved where a section of the wheel passes the output lighttransparently.
 14. A color projection device according to claim 9,further comprising an optical mask.
 15. A color projection deviceaccording to claim 14, wherein the mask has a window of substantiallyelliptical shape.
 16. A color projection device according to claim 14,wherein the mask has a window of substantially pincushion shape.
 17. Acolor projection device comprising three image generators each adaptedto receive a respective color separated digital image signal; and anoptical arrangement such that the separate image generators operate ondifferent regions of the color spectrum to deliver in combination aprojected color image; wherein the optical arrangement is furtherprovided with a lighting mode in which the three image generatorsoperate on the same region of the color spectrum to deliver a highintensity monochrome image beam.
 18. A device according to claim 17,wherein the image generators comprise a digital micro-mirror device. 19.A method for producing an output light beam of desired beam shape,comprising: generating an input light beam having a beam shape defininga larger cross-sectional area than said desired beam shape; employing animage generator to modulate the input light beam so as to maximize thenumber of digital elements of the generator contributing to the lightoutput; and correcting the resultant image beam to the desired beamshape.
 20. A method according to claim 19, comprising processing imagedata input to the image generator to pre-compensate for the correctionof the image beam.
 21. A device according to claim 9, wherein the lightmodulator comprises a digital micro-mirror device.
 22. A colorprojection device according to claim 5, wherein the mask has a window,for defining the beam section, of substantially elliptical shape.
 23. Acolor projection device according to claim 5, wherein the mask has awindow, for defining the beam section, of substantially pincushionshape.